I believe I found a bug in Subscribe2 HTML. I just upgraded to the latest 8.5 and the problem still remains.
When manually sending an email, if I try to add an image that has been rescaled by wordpress... meaning it's a version that has -100x100, -300x225, -920x360, etc.. at the end of the file name, it will not send the email. If I press the Send button or the Preview button, it thinks about it for a second, and then I get redirected to my site's home page. No error message. I believe the problem is in the filename of those images, because for example I wanted to add the following image:
<img src="http://culturedfoodlife.com/wp-content/uploads/2012/02/caldwell-100x100.jpg" />
that won't work, but if I rename the file and change the link to lets say: <img src="http://culturedfoodlife.com/wp-content/uploads/2012/02/caldwell-a.jpg" /> there are no problems. You can use this img link to test out yourself. If I have other images linked in the email that do not have the scaled numbers at the end of the file name, as long as I have at least one, the email messes up